The long term goal of this project is to develop effective strategies that will prevent insulin immunity and autoimmunity. To accomplish this goal, the sites and stages of B lymphocyte development that permit anti-insulin B cells to differentiate and produce antibody will be identified. In contrast to models where the normal immune system deletes or silences autoimmune B cells, autoantibodies to insulin routinely follow administration of autologous hormone. These antibodies may lead to allergic reactions and hormone resistance as well as covert complications that include large birth weight infants and accelerated vascular disease. Spontaneous insulin antibodies may accompany systemic autoimmune disorders and are recognized as part of the autoimmune prodrome of type I diabetes. These observations led to the assertion that the immune system ignores insulin because B cell receptor (BCR) interactions are too few or too weak to induce tolerance. Data on anti-insulin B cell repertoires, however, are not consistent with the concept of true "clonal ignorance". Anti-insulin BCR found in preimmune repertoires are not part of the expressed regions but lineages of insulin binding B cells do not arise, indicating that anti-insulin B cells are censored in stages of differentiation. These observations suggest the hypothesis that insulin autoimmunity arises as a consequence of competing forces that drive clonal expansion of B cells while eliminating self-reactivity. This hypothesis will be tested by using mice that express anti-insulin BCR transgenes that bind insulin with a range of affinities representative of a physiologic repertoire. Three specific aims will (1) determine how the affinity of the preimmune repertoire for endogenous insulin governs the outcome of B cell activation -- tolerance or differentiation; (2) identify the mechanisms that limit expansion of B cells not silenced in the preimmune repertoire; and (3) identify the cell activation events and nuclear transcription pathways that program the phenotypes of B cell differentiation or tolerance. Based on the outcomes of these aims, future strategies may be directed at deletion of low affinity anti-insulin B cells or at inducing clonal elimination in germinal center reactions. These are alternative approaches that may be rationally applied once the stages of B cell development and differentiation that fail to maintain tolerance are identified in normal immune systems and in autoimmune diabetes.